Contains report on one research project.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)U. S. Air Force (ESD Contract AF 19(628)-6066)M. I. T. Sloan Fund for Basic Researc

Muehlner, D. J.

Perry, C. H.

Contains research objectives and reports on one research project.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E)U.S. Air Force (ESD Contract AF 19(628)-6066)M.I.T. Sloan Fund for Basic Researc

Tornberg, N. E.

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Optical and Infrared Spectroscopy

V. OPTICAL AND INFRARED SPECTROSCOPYAcademic and Research StaffProf. C. H. PerryDr. R. P. LowndesGraduate StudentsJeanne H. Fertel J. F. ParrishD. J. Muehlner N. E. TornbergA. IMPURITY-INDUCED ABSORPTION IN PEROVSKITE FLUORIDESThe fundamental lattice vibrations and multiphonon bands in various perovskite flu-orides have been investigated by Perry and Young, 1 ', 2 and the study of the far infrared-1absorption spectra (below 100 cm- ) of these materials was initiated to find antiferro--1magnetic resonance (AFMR) in KNiF 3 . A strong band at 49 cm confirmed a similarline reported by Richards 3 which he assigned to AFMR. The same sample of KNiF 3 wassubjected to magnetic fields of up to 150 kG at the Francis Bitter National Magnet Lab-oratory, M. I. T. A field of this magnitude would be expected to shift an AFMR frequency-1 -1by ~15 cm . No shifts were observed, however, in the strong band at 49 cm or inseveral sharp weaker bands that were also observed (Fig. V-l). KNiF 3 has nearest-neighbor spins oppositely directed along a cube axis, and is particularly interestingbecause it undergoes no measurable distortion below TN (TN= 2 7 5 0 ). Further evidencethat the observed bands in KNiF 3 were not in fact of magnetic origin was found when asecond sample of KNiF 3 , grown at a later time, displayed a considerably different-1spectrum. The strong band at 49 cm was greatly reduced in strength, and the weaker-1bands were not evident (Figs. V-2 and V-3). The temperature dependence of the 49 cmband was found to be opposite to that expected of AFMR. The dependence of the absorp-tion spectrum on the particular sample, and the lack of sensitivity to a strong magneticfield indicate that the observed absorption bands are due only to impurities. Table V-lcompares some of the properties of various perovskite fluorides. Examination of these-1crystals at low temperatures has shown many absorption bands below 100 cm , mostof which are probably impurity-induced. KMgF 3 , a diamagnetic crystal, shows a band-1 -1at 52 cm which seems to correspond to the KNiF 3 band at 49 cm and there are also-1bands located at 64, 73, 88 and 135 cm . The spectrum again depends on the sample,although not as strongly as is observed with KNiF 3 (Fig. V-4). The mixed crystalsystem K(Mg/Ni)F 3 was also investigated, and the frequency of the strongest band,This work was supported principally by the Joint Services Electronics Programs(U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E),and in part by the U. S. Air Force (ESD Contract AF 19(628)-6066), and the M. I. T. SloanFund for Basic Research.QPR No. 89KNiF60c50402 30S201020 30Liq HeLiq N240 50 60 7(WAVE NUMBERS cm80C 90 100Fig. V-i. Transmission of KNiF 3 (sample #1) at liquid-helium and liquid-nitrogen temperature.KNiF #2 (24mm40 50 60 70WAVE NUMBERS crrLIQ. HeLIQ N280 90 100KN F3  (2.4 mm)LIO. He- SAMPLE #1SAMPLE # 230 40 50 60 70 80 90WAVE NUMBERS cm-Fig. V-3.Transmission of KNiF 3 (sample #2) atliquid-helium and liquid-nitrogen temper-ature.QPR No. 89KNiF 3 transmission spectra at liquid-helium temperature, showing comparisonbetween the two samples grown at differ-ent times.70605040302030Fig. V-2.(V. OPTICAL AND INFRARED SPECTROSCOPY)Table V-i. Properties of cubic perovskite fluorides.Crystal Neel Cubic Lattice Constant Temperature at whichTemperature at 300 'K (A) structure changes(-K) (OK)KMgF 3  diamagnetic 4. 000KNiF 3  275 4. 014 remains cubicKZnF 3  diamagnetic 4. 055KCoF 3  114 4. 068 114 (tetragonal)RbMnF 3  82 4. 240 cubic through 20-1-50 cm -1, varied smoothly with the relative concentrations of Mg and Ni. No otherbands could be traced through the whole range of concentration with confidence(Fig. V-5).Three other crystals which were investigated were K(Zn:l%Ni)F 3 , RbMnF 3 and70z70or # 55770< TI- KMg F (I.9mm)0o 1 1 i i 50 70 90 110 130 150WAVE NUMBERS (cm-1)Fig. V-4. KMgF 2 transmission spectra at liquid-helium temperature. The three samplesfrom different sources show almost iden-tical spectra.KCoF 3 (Figs. V-6 and V-7). In each of these crystals absorption bands were foundwhich may be attributable to impurities. (The strongest band shown for RbMnF 3 isthe lowest frequency lattice band). KCoF 3 exhibits a large number of absorptionbands, of varying intensities and widths. It is possible that one of these may bean AFMR band, as no field measurements have been made.-lIt is interesting that the strongest impurity bands found (KNiF 3:49 cm ; KMgF 3 :52-1 -1 -1and 88 cm ; KZnF 3:25 cm , and possibly KCoF 3 :23 cm - ) all show the sameunusual temperature dependence, as shown in Figs. V-1, V-2, V-6, and V-7. Inaddition to broadening with increasing temperature these bands shift to higher fre-quencies, and this type of temperature dependence may be expected from, a veryQPR No. 89(V. OPTICAL AND INFRARED SPECTROSCOPY)mm)40 60 80 100 120 140WAVE NUMBER (cm - )Fig. V-5. Transmission of K(Mgx:Ni 1 -x)F 3 atas a function of composition.40 60 80 100liquid-helium temperatureloosely bound impurity situated on a lattice site. Here, the energy level scheme hasincreasingly spaced levels that are populated when the temperature is raised and thecentroid of absorption moves to higher frequency.Spin wave dispersion in antiferromagnetic KMnF 3 at 4. 20 K has been observed whenusing neutron inelastic scattering. 4 From these results, the magnon frequency at-1 -1q = 0. 8ql A-1 in the [100] direction was ~68 cm . No band in the region of 136 cm(i. e., two-magnon electric-dipole absorption) was observed and it is quite likely thatnone of the bands in any of the antiferromagnetic perovskites can be attributed to two-magnon absorption (compared, for example, with those found 5 in MnF 2 ).N. E. Tornberg has also investigated the Raman spectrum of several of the crystalsat 20°K, but no similar bands were found and there was only an indication of anextremely weak two-phonon spectrum. Again no bands were observed that could beattributed to two-magnon transitions as observed in FeF 2 6Most of the crystals investigated were subjected to spectrographic analysis to obtainsemiquantitative values for the impurity concentrations. These are summarized inQPR No. 89az'K(Zn: I%Ni) F3(1.7mm)SLIQ. He--- LIQ. N2-- I~-NnvNV NN20 30 40 50 60 70 80 90 100RbMnF 3 (1. mm)LIQ. HeLIQ. N2az40 50 60 70 80 90WAVE NUMBERS (cm - )100 120 130Fig. V-6. Transmission of K(Zn:l%Ni)F 3 and RbMnF 3liquid-nitrogen temperature.at liquid-helium andFig. V-7. Transmission of KCoF 3 at liquid-heliumand liquid-nitrogen temperature.20 30 40 50 60 70 80 90WAVE NUMBERS cm-1QPR No. 895040-3020401 --50U40oz 30S20I(V. OPTICAL AND INFRARED SPECTROSCOPY)Table V-2. Impurity concentrations (in parts per million) for theinvestigated fluoride crystals.103-104 10 2-103 10-10KNiF 3 #I Ba, Sr Na Ca, Co, AlKNiF 3 #2 - Na Ca, MgKMgF 3 #1 - Ca NaNiKMgF 3 #557 - Ca, NaNiKMgF 3 #Tl - Ca, Na, FeKMgF 3  Na O, Si, Fe Cl, CaKMgF 3 #5 - - Ca, NaKZnF 3 (1% Ni) Ni Fe Ca, Mg, NaRbMnF 3  - - Ca, K, Na, ZnKCoF 3  - - Ca, Ni, Cu, Mg, Na, ZnMass spectroscopic analysis.Table V-2, and fall approximately within the ranges specified.In all samples Na and Ca are present, and possibly these impurities are the onesresponsible for the main absorptions in KMgF 3 and KNiF 3 . The main difference,however, between KNiF 3 #1 and #2 samples (see Fig. V-3) is the large doping of Ba andSr in #1; these may be responsible for the additional lines seen in the KNiF 3 (#1)spectrum.There is some discrepancy between the mass spectroscopic analysis and the spectro-graphic analysis for the KMgF 3 sample. The mass spectrographic analysis also indi-cates, however, the presence of oxygen and chlorine (not detectable by emissionanalysis), and no doubt all of the samples contain these substitutional impurities.Consequently, although the spectra yield a large amount of information, it is virtu-ally impossible, at this point, to make assignments and attempt theoretical calculations.The materials obviously contain numerous impurities, and unless more pure startingmaterials are available that are then doped with a known impurity, it appears to be ahopeless task to identify any of the bands. Nevertheless, the results show that greatQPR No. 89(V. OPTICAL AND INFRARED SPECTROSCOPY)precaution must be excercised in the interpretation of any weak band observed at low-ltemperatures in this frequency range (10-150 cm- ) where absorptions attributable tosmall concentrations of impurities can be considerably significant.We would like to thank Dr. H. Guggenheim of the Bell Telephone Laboratories, Inc.,and Dr. A. Linz of the Materials Center, M. I. T., for many of the samples. Thispermitted comparisons between samples from different sources, but even this additionalinformation has not aided in the interpretation.D. J. Muehlner, C. H. PerryReferences1. C. H. Perry and E. F. Young, J. Appl. Phys. 38, 4616 (1967).2. E. F. Young and C. H. Perry, J. Appl. Phys. 38, 4624 (1967).3. P. S. Richards, J. Appl. Phys. 34, 1237 (1963).4. S. J. Pickart, M. F. Collins, and C. G. Windsor, J. Appl. Phys. 37, 1054 (1966).5. S. J. Allen, Jr., R. Loudon, and P. L. Richards, Phys. Rev. Letters 16, 463(1966).6. P. A. Fleury, S. P. S. Porto, L. E. Cheeseman, and H. J. Guggenheim, Phys.Rev. Letters 17, 84 (1966).QPR No. 89

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Optical and Infrared Spectroscopy

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